Unaccountable means impossible to explain. An unaccountable delay, for some unaccountable reason, unaccountable behaviour.
It also has a quite different political sense: an unaccountable body is one that answers to nobody, which is a criticism rather than a puzzle. Context settles which is meant, and the two rarely appear near each other.
1adjectiveunaccountableImpossible to explain.
“Her sudden, unaccountable anger surprised everyone at the table.”
unaccountable is a quality.
From un- + accountable.
Etymology from Wiktionary, CC BY-SA